Absence of PD-L1 Protein Expression in Classic Dermatofibrosarcoma Protuberans

Background: This study aimed to assess the prevalence of PD-L1 protein expression in dermatofibrosarcoma protuberans (DFSP) to provide insights into the potential use of immune checkpoint inhibitors. Methods: We retrospectively analyzed formalin-fixed, paraffin-embedded primary DFSP specimens (n = 17). Diagnoses were confirmed by two senior dermatopathologists according to guideline criteria, including diffuse CD34 positivity and storiform spindle cell morphology. All cases represented conventional DFSP without fibrosarcomatous transformation. PD-L1 immunohistochemistry was carried out using a rabbit monoclonal antibody (ab205921, clone 28-8; Abcam). Only membranous staining in viable tumor cells was scored as a tumor proportion score (TPS), where >1% was considered positive. Any cytoplasmic staining without convincing membranous accentuation was not scored. PD-L1 staining in tumor-infiltrating immune cells was recorded separately. Five pleomorphic dermal sarcomas served as positive controls. Results: The median age was 62 years (IQR 55–74); 12 patients were men and 5 were women. The primary sites were trunk (59%), upper extremity (35%), and lower extremity (6%); immunosuppression was present in 18%. By FNCLCC, 82% of tumors were G1 and 18% were G2; no G3 tumors were identified. All DFSPs were PD-L1-negative in DFSP cells (TPS ≤ 1%) and in tumor-infiltrating lymphocytess. Among controls, 3/5 pleomorphic dermal sarcomas were PD-L1-positive with the expected membranous pattern and variable intensity. Conclusions: PD-L1 expression was absent in this cohort of conventional, predominantly low-grade DFSP, suggesting that classic DFSP is generally not an ideal candidate for PD-1/PD-L1-directed checkpoint blockade. These conclusions should not be extrapolated to fibrosarcomatous DFSP or metastatic disease, where PD-L1 expression has been reported. Selective PD-L1 testing may still be warranted in clinically aggressive scenarios (e.g., fibrosarcomatous transformation, unresectable recurrence, or metastasis).